Democrats are at risk of losing control of the Senate this November, after a new poll found that voters prefer a Republican-controlled Congress to check President Barack Obama's agenda when his approval rating is at its lowest point ever.

Just 41% approve of Obama's job performance, according to the new Washington Post-ABC News poll released Tuesday, a new low in his presidency and down from 46% over the first three months of the year. Some of his approval ratings on individual issues are even worse: 42% on handling the economy, 37% on the Affordable Care Act's implementation, and 34% on his handling of the Ukrainian crisis.

50% of respondents said the implementation of the law is going worse than they expected, compared to 41% who said it's going better than they thought. This comes despite the fact that enrollment exceeded original targets, as more than 8 million people signed up for insurance through exchanges set up by the law.

47% say the law is making their health-care costs go up. Just 7% say it's making costs go down, and 41% say their health-care costs have stayed about the same.

By an even wider margin, respondents said they think the ACA is leading to health-care costs increases across the country — 58% said Obamacare is making health-care costs rise, while only 11% said they think costs are decreasing.

By a 44-24 margin, respondents said the law is helping to make the quality of the nation's health-care system worse.

Democrats, however, do enjoy an overall advantage on voter trust. Voters say they prefer Democrats' stances on health care (43-35), immigration (40-34), and the economy (41-38), among others.

By a 40-34 margin, voters also say they trust Democrats to "coping with the main problems the nation faces over the next few years." But 53% of respondents said they'd rather see Republicans in control of Congress, compared with 39% who said they want it controlled by Democrats.